ISO is the unit for the light sensitivity of the camera’s storage medium. For analogue cameras it refers to the film speed, for digital cameras to the sensor speed.
The higher the ISO value, the more light-sensitive is the sensor or film, i.e. the less light is required. This makes it possible to take pictures in the evening and even at night.
Disadvantages of a high ISO setting: Reduction of image quality in the form of blur and noise.